1. Field of the Invention
The present invention relates to dispersion compensating optical fibers that are suitable for use in wavelength division multiplexing (WDM) systems, more particularly to dispersion compensating fibers that are particularly well suited for use in the C-band and L-band operating windows.
2. Technical Background
To meet the ongoing drive for more bandwidth at lower costs, telecommunications system designers are turning to high channel count dense wavelength division multiplexing (DWDM) architectures, longer reach systems and higher transmission bit rates. This evolution makes chromatic dispersion management critical to system performance, as system designers now desire the ability to accurately compensate dispersion across entire channel plans. Typically, the only viable broadband commercial technology to battle dispersion has been dispersion compensating fibers (DCF) modules. As DWDM deployments increase to 16, 32, 40 and more channels, broadband dispersion compensating products are desired. Telecommunications systems presently in place include single-mode optical fibers which are designed to enable transmission of signals at wavelengths around 1550 nm in order to utilize the effective and reliable erbium fiber amplifiers.
One such fiber, LEAF optical fiber, manufactured by Corning Inc., is a positive nonzero dispersion shifted fiber (+NZDSF), and has become the optical fiber of choice for many new system deployments due to its inherently low dispersion and economic advantage over conventional single mode fibers.
With continuing interest in going to even higher bit rates ( greater than 40 Gbs), Ultra-long reach systems ( greater than 1000 km) and optical networking, it will become imperative to use DCFs in networks that carry data on Non-Zero Dispersion shifted fiber (NZ-DSF) as well. The early versions of DCF""s, those developed for single mode fibers, when used in combination with NZ-DSF fibers effectively compensated dispersion at only one wavelength. However, high bit rates, longer reaches and wider bandwidths require dispersion slope to be compensated more exactly. Consequently, it is desirable for the DCF to have dispersion characteristics such that its dispersion and dispersion slope is matched to that of the transmission fiber it is required to compensate. The ratio of dispersion to dispersion slope at a given wavelength is referred to as xe2x80x9ckappa (xcexa)xe2x80x9d. Kappa changes as a function of wavelength for a given transmission fiber. Hence, it is equally important that as we migrate to Ultra broadband networks that the kappa value of the DCF is matched to that of the transmission fiber at more than one wavelength.
It would be desirable to develop alternative dispersion compensating fibers, particularly ones having the ability to compensate for dispersion of non-zero dispersion shifted fibers and other positive dispersion optical fibers over a wide wavelength band around 1550 nm.
One aspect of the present invention relates to a dispersion slope compensating optical fiber which comprises a core refractive index profile which is selected to result in a fiber which exhibits negative dispersion and dispersion slope at 1550 nm, and a kappa value greater than 35. The kappa (xcexa) value of a DC fiber is defined herein as:
xcexa=(DDC)/(DSlopeDC) 
where DDC and DSlopeDC are the chromatic dispersion and dispersion slope of the DC fiber, respectively, the dispersion value being measured at 1550 nm, and the dispersion slope being measured over the wavelength range of 1530 to 1560 nm.
The negative dispersion slope of the fibers of the invention is less than xe2x88x921.0 ps/nm2/km, over the wavelength range 1530 to 1560 nm. In one preferred embodiment, the dispersion slope is between about xe2x88x921.5 and xe2x88x923.0 ps/nm2/km, and in another preferred embodiment, the dispersion slope is between about xe2x88x921.8 and xe2x88x922.5 ps/nm2/km over the wavelength range 1530 to 1560 nm.
The fibers of the present invention also exhibit a very negative dispersion at 1550 nm, i.e., less than xe2x88x9230 ps/nm/km. The preferred fibers of the present invention exhibit a dispersion at 1550 nm which less than xe2x88x9250 ps/nm/km, more preferably less than xe2x88x9270 ps/nm/km, and most preferably less than xe2x88x92100 ps/nm/km.
Preferred fibers in accordance with the present invention are capable of exhibiting a kappa value at 1550 nm between 40 and 100 or more. The desired kappa may thus be selected depending on the long haul fiber that is to be compensated. For example, one preferred embodiment relates to fiber made in accordance with the invention which exhibit a Kappa between about 40 and 60 at 1550 nm. This preferred embodiment is especially useful for compensating the dispersion created in the C-band (e.g., 1530-1565) by an optical communication system which utilizes LEAF(copyright) optical fiber.
Fibers disclosed herein may also be used in the L-band (1565-1625 nm). In particular, we have found that insertion losses are achievable which are suitable for making the fibers of the present invention suitable for use in the L-band, i.e., less than 1 dB per kilometer. The fibers which are L-band compatible exhibit a xcexa at 1590 nm which is also greater than 50, more preferably greater than 70. In one preferred embodiment, these fibers exhibit a xcexa at 1590 nm which is between about 80 and 100. This preferred embodiment is especially useful for compensating the dispersion created in the L-band by an optical communication system which utilizes LEAF optical fiber. Thus an overall preferred range for C and L band compensation is between xe2x88x9240 and xe2x88x92150, and more preferably between xe2x88x9240 and xe2x88x9290.
All of the above described properties are achievable utilizing fiber having a refractive index profile which comprises a central segment having a relative refractive index xcex941, a second annular segment surrounding the central core segment having relative refractive index xcex942, a third annular segment which surrounds said second segment having relative refractive index xcex943 and a cladding layer having relative refractive index xcex94c, wherein xcex941 greater than xcex943 greater than xcex942 and:   Δ  =                    (                              n            1            2                    -                      n            c            2                          )                    2        ⁢                  n          1          2                      xc3x97    100  
Preferably, the refractive index profile is selected so that the ratio of the refractive index xcex94 of the second core segment to that of the first core segment (xcex942 /xcex941) is greater than xe2x88x920.4. More preferably, the ratio of the deltas of the second segment to the first segment xcex942/xcex941 is greater than xe2x88x920.37. Also, preferably, xcex941 greater than xcex943 greater than xcex94c greater than xcex942.
If the negative dispersion slope of the fiber is made less than xe2x88x920.08 ps/nm2/km, the fibers will have particular utility for compensating the dispersion for large effective area (greater than 50, more preferably greater than 60, and most preferably greater than 65) nonzero dispersion shifted fibers. One such fiber, Corning""s LEAF(copyright) fiber, is a optical fiber having a zero dispersion wavelength outside the range of 1530-1565, and an effective area greater than 70 square microns. LEAF fiber""s larger effective area offers higher power handling capability, higher optical signal to noise ratio, longer amplifier spacing, and maximum dense wavelength division multiplexing (DWDM) channel plan flexibility. Utilizing a larger effective area also provides the ability to uniformly reduce nonlinear effects. Nonlinear effects are perhaps the greatest performance limitation in today""s multi-channel DWDM systems. The dispersion compensating fibers disclosed herein are exceptional in their ability to compensate for the dispersion of NZDSF fibers, in particular Corning""s LEAF fiber. LEAF optical fiber nominally exhibits an effective area of 72 square microns and a total dispersion of 2-6 ps/nm/km over the range 1530-1565.
Additional features and advantages of the invention will be set forth in the detailed description which follows, and in part will be readily apparent to those skilled in the art from that description or recognized by practicing the invention as described herein, including the detailed description which follows, the claims, as well as the appended drawings.
It is to be understood that both the foregoing general description and the following detailed description are merely exemplary of the invention, and are intended to provide an overview or framework for understanding the nature and character of the invention as it is claimed. The accompanying drawings are included to provide a further understanding of the invention, and are incorporated in and constitute a part of this specification. The drawings illustrate various embodiments of the invention, and together with the description serve to explain the principles and operation of the invention.